Inside Health

Q & A; Magnetic Metals

By C. CLAIBORNE RAY

Published: May 16, 2006

Q. I have a titanium screw in my skull for a bone-anchored hearing aid. Does this mean I cannot have an M.R.I.? What elements make something attractive to a magnet?

A. You can still have magnetic resonance imaging with a titanium implant. In fact, titanium is used for many medical implants because it does not interfere with CT scans and M.R.I.'s the way steel implants can, implant manufacturers say.

Titanium may obscure specific areas of an image, but it is not affected by even strong magnetic fields.

Not all metallic elements are attracted to magnets, and most orthopedic implants are made of nonmagnetic alloys. The four strongly magnetic elements are iron, nickel, cobalt and gadolinium.

The strength of magnetic attraction depends on the way moving electrons are aligned in the atoms of a substance, sometimes forming minuscule internal magnets.

In some substances, the tiny magnetic fields cancel one another out or are always randomly oriented. In materials attracted to magnets, the internal magnets within the material line up with the magnetic field of the magnet. That gives the material a magnetic field, and it is then pulled to the magnet.

The fields vary with temperature, becoming stronger as it grows colder and disappearing at high enough temperatures.

In the four ferromagnetic elements, the internal magnetic fields can stay aligned even at normal temperatures. C. CLAIBORNE RAY